Power integrated circuit, or high voltage integrated circuit, is an important branch of modern electronics. It provides new type of circuits for various power converters and energy treatment device, capable of high speed, high integration density, low power consumption and radiation-hardness. Power integrated circuit is widely used in power control systems, automotive electronics, display drivers, communication devices, lighting industry, national defense, aerospace, and many other important areas. Because they are widely used, high voltage devices being the core of power integrated circuit are subject to strict requirements.
A power integrated circuit includes high voltage power transistors, conversion controllers, single-chip logic devices, etc. There high voltage power devices and low voltage logic devices have to be integrated on one chip. Silicon on insulator (SOI), being an ideal isolation material, can isolate high power modules from low power modules, and isolate high voltage devices from low voltage devices as well. Therefore, SOI eliminates electrical interference and simplifies device structures in a power IC. Furthermore, the surface of isolation regions in SOI is smaller than junction isolation regions, thus using SOI power IC can shrink chip area and reduce parasitic capacitance, making device scaling relatively easy.
IC products with SOI power devices operating at higher than 600V are widely used in fluorescent lamps, switching power supply controllers, and other devices. However, the vertical breakdown voltage of a conventional SOI high voltage device is lower than that for bulk silicon high voltage power devices, because the buried oxide layer (BOX) prevents the substrate to expand to the depletion layer). The SOI power IC devices designed for high voltage below 200V is usually easier than those products designed for high voltage above 600V.
The thickness of the top silicon film in SOI affects the maximum breakdown voltage. When the silicon film is thick, (typically greater than 1 micron), the vertical breakdown voltage increases with silicon film thickness. However, when the silicon film is thin (typically less than 1 micron), the vertical breakdown voltage increases as silicon film thickness decreases. Currently, the most effective techniques for manufacturing above 600V SOI power devices are: first using ultra-thin top silicon (0.2 to 0.5 microns thick) to shorten the ionization integration path to improve maximum vertical breakdown voltage, and second applying linear drift doping to achieve uniform distribution of lateral electric field.
However, due to existing process' limitations, the thickness of the top silicon film in SOI is usually greater than 1 micron. In order to achieve 0.2-0.5 micron thin silicon film, local oxidation of silicon (LOCOS) process is often adopted. But there are some problems with the LOCOS process which requires a long time to oxidize silicon locally for forming about a nearly 2 micron oxide layer, and the formed oxide layer is higher than the top silicon film of about 1 micron thick. In FIG. 1, the oxide layer fabricated 14 in the local region of a SOI substrate (including bottom silicon layer 11, BOX 12 and top silicon layer 13) is higher than the surface of SOI substrate. The higher part of the oxide layer will affect the precision of the follow-up photo-etching process, although the poly silicon gate can be extended on it to control the electric field of the drift region.
In addition, the SOI high voltage control chips usually include high voltage devices and low voltage devices, and the isolation structures between high voltage devices and between high voltage device and low voltage device utilize insulation trenches, while the isolation structures between low voltage devices utilize LOCOS structures.
Therefore, there is an urgent need to effectively combine the trench process and the LOCOS process on a single high voltage SOI chip.